The spectral distribution of conventional high-pressure sodium vapor lamps has concentrated radiation in the area of the greatly broadened and self-reversed main resonance lines. Since this spectral range is close to the maximum eye sensitivity, such lamps have very high light yields, for which, depending on the electrical power, values between 75 1m/W for 35-watt lamps and 150 1m/W for 1000-watt lamps are obtained. Unfortunately, the color temperature (T.sub.c) of approximately 2000 K and the general color rendering index (R.sub.a) of about 20 are comparatively low, so that such lamps cannot be used for more demanding lighting applications.
Numerous attempts have been made in the past to improve the color properties of high-pressure sodium vapor lamps by structural techniques or by changing the system of operation. Some such attempts increase the temperature of the cold spot in the burner in order to obtain a better spectral distribution for color quality at a higher sodium partial pressure. The improvements achieved are indicated to be R.sub.a values up to 80 and color temperatures up to 2500.degree. K., but they entail a reduction of the light yield by as much as 40% and a sometimes drastic reduction of useful life (see, for example, J.A.J.M. van Vliet, J.J. de Groot; High-pressure sodium discharge lamps, IEE Proc. 128 (1981) 41). Even the use of red-absorbing filter glasses (DE 2 711 733) only partially helps these deficiencies, since the raising of the color temperature that is achieved entails great light yield losses.
Other attempts to supplement the radiant content lacking in the blue-green range of conventional high-pressure sodium vapor lamps by expanding the luminous system have been described in older patents (U.S. Pat. No. 3,521,108, GB 1,192,094, GB 1,280,370). In these references the addition of thallium, cadmium, zinc or lead or combinations of these elements has been proposed, and yet these efforts have not resulted in a commercially salable lamp. The reasons for this are to be seen, for one thing, in the plasma temperatures typical of such lamps, which with axis levels of about 4000.degree. K. offer only very unfavorable conditions for the excitation of radiation processes at higher energy levels. Occasionally, too, high-pressure sodium vapor lamps with the addition of cadmium have been described (DE 2 032 615) in which cadmium functions as a buffer material and is intended to contribute to the stabilization of the lamp parameters. Additions of cadmium, thallium, zinc, lead, tin and bismuth have been disclosed (DE 2 707 203) in order to reduce the mercury content in high-pressure sodium vapor lamps in the interest of reducing environmental harm and providing improved resistance of the filler substances to air. Another possibility for exercising a controlled influence on the color properties of high-pressure sodium vapor lamps involves using a time-modulated energy input that is different from operation at power mains frequencies. In this case the energy is fed to the lamp in the form of pulses, while the average power and hence the temperature of the discharge vessel are not raised, although the current level in the pulse is definitely above the levels otherwise commonly used. The lamp power input via the pulses over the total amount of time (N) and the power converted during the pulses (N) are related by the duty cycle (.DELTA.) according to the equation N=.tau.. N, and the product of the pulse length (t) times the pulse series frequency (f) gives: .tau.=g . t. A high-pressure sodium vapor lamp operated by this method has been described in the patent literature (U.S. Pat. No. 4,137,484, DE 2 657 824), f having been varied between 500 and 2000 Hz and .tau. between 0.1 and 0.3. In this manner it was possible, by the excitation of a correspondingly higher electronic level of the sodium and of the mercury usually serving as buffer gas, to raise the color temperature from 2300 to 2600.degree. K. These values are still too low for the use of such light sources in interior lighting, especially residential lighting, since here color temperatures of 3000.degree. K. and more are desirable. Furthermore, the experiments showed that the visual efficiency of lamps operated in this manner diminishes below a certain duty cycle value at a given pulse series frequency. For example, the light yield of the 150-watt lamp described in patent DE 2 657 824 decreases for color temperatures above 2700.degree. K. to below the level achievable by operation at the mains frequency (by about 5 1m/W for each increase of 120.degree. K. in the color temperature), so that pulsed operation of such lamps is unfavorable in this range. The same conclusion has been reached by other authors (N. Brates, E.F. Wyner: Pulsed Operation of a High Pressure Sodium Lamp; J. Illum. Eng. Soc., New York 16 (1987) 50), who have performed similar studies on conventional 100-watt lamps, though for more or less randomly selected operating conditions (.tau.=0.015, f=150 Hz). The results of these experiments confirm the not-unexpected increase in color temperature for the chosen duty cycle (during the pulse, T.sub.c levels no higher than 4300.degree. K. are achieved), but in time averaging they show only moderate improvements of the color properties, while the light yield does not achieve the levels achieved in the conventional mode of operation at any time during the discharge. The fact that the time-averaged levels of T.sub.c and R.sub.a could be improved only slightly in comparison with the steady state is due to the relatively high energy transformation between the pulses, which amounted in the present case to about 50% of the total energy input. In this manner the plasma's emission is of low effectiveness over relatively long periods, so that the effect at which the pulsed input is aimed is jeopardized.
Since the direct pulse supply is provided, for example, by connecting charge capacitors, certain time/power events result from the capacitor discharge curves, plasma characteristics, plasma dynamic and electrode factors, which sometimes depart considerably from any form that is desirable from the viewpoint of plasma physics and illumination technology.
A modified variation of pulsed input is discussed in DE-OS 3636901 and DE-OS 3641070. Here, a high-frequency current of 20 to 50 kHz, for example, is superimposed upon bipolar current pulses with a scan ratio of 0.2 to 0.6. This superimposition is intended to reduce the flicker observed in pulse-driven high-pressure lamps without exercising any direct influence on the illumination data. At a duty cycle of 0.3, the color temperature for 30-watt lamps is 2700.degree. K. in this case.
Operation of high-pressure sodium vapor lamps containing at least one other metal vapor in addition to sodium with unipolar pulses of 50 to 23000 Hz and with a duty cycle between 0.08 and 0.8 has also been disclosed for the purpose of preventing separation phenomena due to cataphoresis (DE 2 729 052).
It has generally been assumed that the known methods for improving the color properties of high-pressure sodium vapor lamps result in more or less great losses in the light yield. The reasons for this situation lie in an increased self-absorption of the resonance lines to the extent that the color improvement is achieved by increasing the sodium partial pressure or, as in the case of the pulsed high-pressure lamp, in a redistribution of the energy to radiation processes with a low illumination efficiency. On the other hand, however, precisely the spectral distribution and hence the light yield are optimized by the technically achieved plasma state and by the luminous system used. The operating regime proposed in DE patent 2 657 824 in connection with the pulsed operation of high-pressure sodium vapor lamps only partially takes this into consideration and therefore results in light-yield losses at higher color temperatures.